1. Field of the Invention
The present invention relates to a method for forming a silicon oxide film, a plasma processing apparatus and a storage medium, and in particular relates to the method for forming the silicon oxide film, plasma processing apparatus and storage medium, each applicable to the case of forming the silicon oxide film used as an insulating film in a process for manufacturing, for example, various semiconductor devices or the like.
2. Background Art
In the process for manufacturing the various semiconductor devices, the silicon oxide film, such as SiO2 or the like, which is used as the insulating film, such as a gate insulating film of, for example, a transistor, is formed. The method for forming such a silicon oxide film can be broadly divided into a thermal oxidation process employing an oxidation furnace and/or RTP (Rapid Thermal Process) apparatus and a plasma oxidation process employing a plasma processing apparatus. For instance, in a wet oxidation process using the oxidation furnace, which is one aspect of the heating process, a silicon substrate or substrates are first heated up to a temperature above 800° C., and then exposed to an oxidizing atmosphere in a WVG (Water Vapor Generator) apparatus adapted for generating steam (H2O) by combusting hydrogen with oxygen. In this manner, the surface of each silicon substrate can be oxidized, thereby forming the silicon oxide film thereon.
Meanwhile, in the plasma oxidation process, a processing gas containing an argon gas and an oxygen gas is used, with an approximately 1% flow rate of oxygen. Specifically, this plasma oxidation process is performed by applying plasma, which has been generated in a chamber under pressure of 133.3 Pa while being excited by a microwave, to each silicon surface. As used herein, such plasma excited by the microwave will be referred to as “microwave excitation plasma.” This process can facilitate control of film thickness and form the silicon oxide film having relatively high quality (see, e.g., Patent Document 1).
Patent Document 1: WO2004/008519
Recently, the thermal oxidation process has been considered as a method that can form the silicon oxide film with higher quality. However, this process should be performed at a considerably high temperature exceeding 800° C. Therefore, this thermal oxidation process may cause an oxidation rate to be excessively elevated and hence unduly increase the film thickness, thus making it quite difficult to control the film thickness, especially in the case of forming a highly thin film. For instance, each corner portion of the silicon surface having a concavo-convex shape may tend to be conspicuously rounded, and it is difficult to control a degree of increase of the film thickness in such a rounded portion. Furthermore, such a thermal oxidation process is likely to increase thermal budget and have negative impact, such as deformation or warp, on each silicon substrate, due to thermal stress.
Meanwhile, the plasma oxidation process described in the above Patent Document 1 is performed at a relatively low temperature of approximately 400° C., thereby substantially avoiding such undesired increase of the thermal budget and warp of each substrate that may be caused by the above thermal oxidation process. In addition, due to the plasma process performed under the condition of approximately 1% flow rate of O2 and 133.3 Pa processing pressure (for convenience, such a condition will be, hereinafter, referred to as a “lower-pressure and lower-oxygen-concentration condition”), a significantly high oxidation rate can be achieved. Besides, this plasma oxidation process can successfully control the rounded shape of each corner portion that will be generally formed in the case of oxidizing the silicon surface having the concavo-convex shape. Moreover, this plasma oxidation process can achieve formation of a highly thin silicon oxide film on the surface having such a concavo-convex shape, leading to positive suppression against occurrence of a leak current from such a surface.
However, when the plasma oxidation process is performed under the lower-pressure and lower-oxygen-concentration condition as described above and if a pattern formed on a surface of an object to be processed includes dense sites and scattered sites, such as closely aggregated lines, relatively opened spaces and the like, partial pressure of oxygen tends to be lowered in each dense site, resulting in a difference in a rate of formation of the silicon oxide film, between such dense sites and scattered sites in the pattern. Therefore, the thickness of the oxide film formed on the silicon is likely to be thinned in each dense site, as such making it difficult to form the silicon oxide film, with a uniform film thickness. Additionally, such variations in the film thickness in particular sites of the silicon oxide film formed on the silicon surface having the concavo-convex shape may tend to cause serious degradation of reliability of the semiconductor device having this silicon oxide film as the insulating film thereof.